This invention relates to cementitious compositions containing Portland cement and fly ash. More particularly, this application concerns novel Portland cement concrete compositions containing relatively low proportions of cement together with relatively high proportions of a select fly ash component, which compositions when mixed with water and allowed to set harden into masses exhibiting unexpectedly high compressive strengths.
Of all the hydraulic cements, Portland cement enjoys the most widespread use for constructional purposes at the present time. Portland cement is produced by heating to fusion mixtures of argillaceous and calcareous materials and thereafter grinding the "clinker" produced, in the presence of up to about five percent gypsum, to a fine powder. Mixtures of Portland cement with a particulate aggregate such as limestone and sand form a "structural" concrete which is unsurpassed as a strong and durable building material.
Fly ash is a well known material generated as a by-product in the combustion of coal as a fuel in power plant boilers. The American Concrete Institute Publication "SP-19" defines fly ash as "The finely divided residue resulting from the combustion of ground or powdered coal and which is transported from the firebox through the boiler by flue gas". The recent growth of coal fired power due to fuel oil shortage and price increases has resulted in an abundance of fly ash with consequent active promotion of uses for the rapidly accumulating solid waste material.
Being finely divided and highly siliceous, fly ash is a well known pozzolan which forms a slowly hardening cement with water and hydrated lime. A pozzolan is defined in the American Concrete Institute Publication "SP-19" as "A siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties". This chemical reaction between the pozzolan and the calcium hydroxide is often referred to as the "pozzolanic reaction". While the "pozzolanic reaction" has been studied, the mechanisms involved are not completely understood. For instance in a report published in June, 1976 by the U.S. Department of Transportation, No. FHWA-IP-76-16 entitled "FLY ASH AS A CONSTRUCTION MATERIAL FOR HIGHWAYS", it is stated that the pozzolanic reaction, although still not completely understood, involves the silica and alumina compounds in the fly ash, as well as any free lime which may be present.
The art has recognized that useful cementitious compositions can be prepared by combining in the presence of water Portland cement and pozzolans such as fly ash. For example, as detailed in the aforementioned published report of the Department of Transportation (DOT), Portland cement may be added to lime-fly ash-aggregate base and sub-base courses in rigid pavement systems as a substitute for all or part of the lime to accelerate the rate of strength gain of the mixtures. On the other hand it is well known to add fly ash and other pozzolans to Portland cement compositions. Commercial Portland cements containing appreciable amounts, 15 to 40 percent, of pozzolan such as fly ash are known in the art as "Type 1P" cements. Moreover, Portland cement concretes have been prepared containing fly ash to increase workability and pumpability, to control bleeding, reduce shrinkage, etc. The major objective however sought by the addition of fly ash to Portland cement concretes in the U. S. has been for the enhancement of compressive strength of the hardened concrete. In such concretes the fly ash has been added as a replacement for the cement and/or sand, the pozzolanic fly ash reacting with the lime produced as the result of the hydration reaction between the Portland cement component and water to give additional strength. Such "pozzolanic reaction" of the fly ash increases the strength of the concrete, especially at ages of 28 days or more. The cementitious compounds produced by the "pozzolanic reaction" are believed to fill the pores and capillaries that are normally present in concrete and as a result the concrete containing fly ash also becomes less permeable to water and salt solutions.
The amount of fly ash in practice added to Portland cement concrete has varied depending upon the desired end properties of the concrete. Generally, when a given Portland cement concrete is redesigned to include fly ash, between 10 and 30% of the Portland cement, by weight, is replaced by a volume of fly ash which ranges from that equal to that of the cement removed to that equal to two times that of the cement removed. One-hundred pounds of a typical Type I Portland cement will produce enough calcium hydroxide during its reaction with water to react with about 20 pounds of a typical fly ash. The amount of fly ash, therefore, over and above this 100 cement to 20 fly ash weight ratio acts essentially in a physical way to affect bleeding, workability and heat of hydration. When the amount of fly ash used in concrete is equal in volume to the amount removed and the cement substitution exceeds about 20%, the compressive strengths of the set concrete are lower as compared to a reference non-fly ash-containing concrete, at all ages, and for this reason substitution beyond the approximate 20% level in concrete has been avoided in practice.
Concretes however wherein fly ash has been substituted beyond the 20% level have been suggested in the art. For example, in U. S. Pat. No. 2,250,107 to John S. Nelles, assigned to the Detroit Edison Company, Detroit, Michigan, Portland cement concretes are described wherein fly ash is added in substitution for part or all of the fine aggregate such as sand conventionally used as the fine aggregate. The fly ash is said to be added in greater proportions than previously added, i.e., at least equal to the amount of cement by volume, with a range of from 1.5 to 2.5 times the amount of cement being preferred. The resulting concretes are said to be stronger than comparable concretes having identical Portland cement content, to be lighter in weight, and to possess better workability and flexibility. Also, in British Patent No. 940,692 to Fonbonne, Portland cement compositions comprised of mixtures of cement and fly ash are described, the proportion of fly ash constituting at least two thirds by weight of the cement. In contrast to prior art cement-fly ash compositions wherein the proportion of fly ash was limited, the hardened compositions of British 940,692 containing the large amounts of fly ash are said to have good compressive strengths after 7 and 28 days. According to the British patent, the prior art cement-fly ash compositions limited the amount of fly ash to 20 percent by weight, since it was found that increased proportions beyond this decreased the strength of the hardened cement. The high fly ash content cement compositions of British Pat. No. 940,692 are said to increase in strength at a greater rate than conventional cements having similar strength at 28 days after setting. According to the British patent it was found necessary to crush the fly ash as uncrushed ash would not produce an effective cement composition.
It is also known in the art that the chemical compositions of fly ashes from coals from differing geographical locations sometimes vary significantly as does their pozzolanic activity. According to the aforementioned DOT report No. FHWA-IP-76-16, the extent and rate of pozzolanic reaction involving fly ashes is a function of several factors including quantity of stabilizer (free lime or cement), total silica and alumina in the fly ash, etc. The DOT report also states that fly ashes having large amounts of free lime (as indicated by, although not equal to, the CaO content) tend to be very reactive and probably exhibit some degree of self-hardening. The report describes compositions stabilized through the pozzolanic reaction by the addition thereto of lime and/or cement, and/or aggregate, particularly useful in constructing pavements, stabilizing soil, embankments, structural backfill, and in forming grouting compositions.
In U. S. Pat. No. 3,634,115 to Leonard John Minnick, a new fly ash is described which is "sulfopozzolanically" reactive. The fly ash is said inter alia to have a high sulfate and a high calcium oxide content rendering it useful for preparing stabilized load-supporting compositions having superior qualities. The new fly ash is produced by injecting lime into the boiler under particular conditions during combustion of the coal. The novel fly ash according to the patent gives especially good results when combined with lime and aggregate, for example, to provide a sub-base or sub-roadway course, although other materials such as Portland cement and ordinary fly ash can be used with the inventive fly ash according to the patent.